I know that I have failed a dialog check that resulted in the death of a tiefling child, so it would seem that this imagined immunity from death is just that, imagined. That's not including references made in this thread that I haven't personally seen, just the one I have, and already I'm left thinking "Are we playing the same game".
As I recall, you have the option of agreeing that death is a good punishment because she's a thief. I might be misremembering, but I know that some of the dialogue is fairly hard on the tiefling's chances.
